In general, in a wire electric discharge machining, when a board thickness of a workpiece is within a range of 40 to 60 mm, a wire breaking limit of a wire electrode is the highest, so that it is possible to set a machining condition having the highest machining energy.
This is because, although there is a limit on the machining current to be supplied to the wire electrode, as the workpiece becomes thicker, a current density per a unit wire electrode length becomes smaller, so that the wire breaking limit is increased. In the meantime, when the board thickness exceeds a threshold value, the effects of removing machining powders and cooling the wire electrode by a machining liquid are deteriorated, so that the wire breaking limit is lowered and thus the wire may be broken down.
In other words, as described above, the inputtable energy becomes maximum when the board thickness is within the range of 40 to 60 mm, and decreases as the board thickness becomes more distant from the range.
In machining the workpiece having different board thickness with a wire electric discharge machining apparatus, when the machining condition is set in correspondence to a board thickness portion having a high wire breaking limit, the wire breaks down when machining a board thickness portion having a low wire breaking limit, and when the machining condition is set in correspondence to a board thickness portion having a low wire breaking limit, the machining speed is lowered when machining a board thickness portion having a high wire breaking limit.
Accordingly, in order to efficiently machine the workpiece without the wire breaking by the wire electric discharge machining apparatus, it is necessary to set an appropriate machining condition in accordance with the change of the board thickness of the workpiece. Patent Document 1 discloses a technology of obtaining a board thickness of a workpiece by calculating a ratio of machining speed and machining energy and selecting appropriate machining conditions in accordance with the calculated board thickness, thereby efficiently performing an electric discharge machining.